Power amplifiers are well-known components used for amplifying electric signals at the signal level, viz. signals at a usually low voltage level. Such a low voltage level can for instance be ±2 volt, ±15 volt or 0 to 5 volt, i.e. the voltages at which ordinary electronic circuits operate. The power amplifier serves to amplify such a low voltage up to a considerably increased power potential. The latter power potential is obtained by intensifying the low voltage to an increased voltage, i.e. by upscaling the low voltage with the result that the signal level is separated from the power level.
Usually, the power amplifiers are classified in class A, class B, class AB, class C and class D amplifiers. A power amplifier classified in class A, class B, class AB or Class C is based on the linear, active area of a transistor. Therefore, the power amplifier can be considered an ideal, controlled voltage source connected in series to an internal resistance, the load of said power amplifier being connected in series to said internal resistance. Elementary circuit calculations show that a rather significant portion of the power supplied by the ideal voltage source is allocated to the internal resistance and thereby not to the load. In theory, it is possible to achieve a maximum efficiency, i.e. the relation between the power received and the power delivered by the power amplifier, of almost 80% by means of a class B amplifier, but in practice, the efficiency does not usually exceed 60%. The power loss is mainly allocated in the power amplifier in form of heat, and said heat must be carried away such as for instance by way of cooling. Usually, the voltage source is not ideal, and it depends on the desired power level, the extent and character of the load as well as on the frequencies involved.
When an input signal with a known frequency content is compared with the resulting output signal, and when said output signal is subjected to a frequency analysis, differences are inevitably found in the relation between the amplitudes of the individual frequencies and the frequencies found in the signal. In addition, these relations change in response to the load on the power amplifier. Therefore, it is possible to indicate a figure for the distortion of the power amplifier versus the frequency and the power, and this figure represents a quality figure for the power amplifier which can be used for determining the class of said power amplifier.
The power amplifiers of the classes A, B, AB and C have been constructed for many years, and the advantages and limitations thereof are well-known as well. However, the power amplifiers of the class D differ from the other power amplifiers by the output transistors thereof being used as switches or electronic switch elements. Most of the internal losses found in a linear amplifier are caused by the fact that the operating point of the transistors used is found in the linear area. This linear area shows a rather significant potential drop across the transistors while a rather significant current passes through said area. On the other hand, when the transistors are always “switched on”, i.e. a low potential drop applies across the transistors while the current through said transistor differs from 0; or when said transistors are “switched off”, i.e. the potential drop across said transistor is significant while the current through the transistor is very low, almost 0; then the resulting power loss in the transistors is always low. Furthermore, the only period involving a significant power loss is the period during which the transistor changes from being switched on to being switched off, i.e. when both the current and the voltage differ from zero. Now, when the transistor is switched on and off at a high speed, viz. at a high frequency, the resulting voltage is a square voltage. When the relation between the switched on and off periods of the transistors is varied, the continuous average value of the resulting voltage represents a predetermined value. The latter pre-determined value can be controlled in such a manner that it is possible to ensure that said value corresponds to the input signal to the power amplifier. Accordingly, it has now been rendered possible to structure a power amplifier presenting a significantly improved efficiency compared to the hitherto known linear amplifiers.
A power amplifier operating according to the above principle is often called a switch-mode power amplifier or a pulse width-modulated power amplifier. Such an amplifier implies that a control signal must be generated for the transistors in order to switch said transistors on and off in response to the input signal. The control signal is for instance generated by a comparison of the input signal with a triangular voltage, and when the triangular voltage presents a value lower than the input voltage, a control signal is generated for the transistors reading that said transistors must either be switched on or off. When the triangular voltage assumes a value higher than the value of the input signal, a second control signal is generated for the transistors, said second control signal presenting a value opposite the value of the preceding signal. The frequency of the triangular voltage is often called the change-over frequency or the switch-frequency. This switch-frequency depends inter alia on the intended use of the power amplifier. When it is a question of a class D amplifier used for playing for instance music, an advantage is found in using the frequency characteristics of the human ear as a starting point when the frequency is to be chosen. The maximum audible frequency for the human ear is usually approximately 20 kHz, and accordingly an advantage is found in choosing a switch-frequency which is considerably higher than said frequency, as an audible howl, a humming or hissing sound is otherwise found in the resulting sound reproduction. Therefore, the resulting output voltage is a box-shaped or pulse-shaped voltage where the width of the individual pulses varies in response to the input signal.
Optionally, the output voltage (scaled down) and the input voltage may be input to an integrator, which integrates the voltage difference between the two signals. The resulting voltage signal is a triangular waveform with a changing frequency. This signal is then fed to a comparator with hysteresis. The voltage from the comparator is a square waveform, where the pulse width is dependent on the input voltage. The advantage is that no additional dedicated circuitry is required for producing the triangular waveform as described in the previous section. The frequency of the waveform, however, is dependent on the input voltage and may become very low, when the input voltage is near or at maximum.
Usually, the switch-frequency used is considerably higher than the frequency maximum to the human ear, and accordingly this switch-frequency does not usually result in audible nuisances. However, the rapidly alternating current and voltage can easily cause a radio-frequency interference, and therefore it is often necessary to include a filter in order to avoid such an interference. Often a second order filter is used where an inductance is coupled in series to the output of the power amplifier and where a capacity is connected to the end of the inductance adjacent the output and to the frame reference. Therefore, the resulting output voltage complies rather well with the input signal.
However, the output filter involves complications. In particular, when the power amplifier is unloaded, the resonance frequency of the filter can be excited, and as a result of the poor attenuation of the filter, viz. a high Q-factor, a rather significant voltage rise can be applied to the output. Such a voltage rise is problematic as it can cause damages to the power amplifier. This problem has often been solved by slightly oversizing the components of the power amplifier and/or by providing the output of said power amplifier with a load resistor. This load resistor is permanently coupled between the positive and negative poles of the output with the result that it provides an attenuation of the resonance of the output filter. This load resistor must necessarily be of such a size that it is able to efficiently attenuate the resonance of the output filter without causing a too high additional loss in the power amplifier. The load resistor is usually a cost-intensive component, and accordingly an advantage is found in avoiding the use of such a load resistor. Usually, the power amplifier is not used in the unloaded state, and therefore the use of a load resistor in form of a loading impedance, such as an RC-element with a high resistance, minimizes the power loss.
As mentioned above it is necessary to use an output filter in order to make the input signal comply well with the resulting output signal. However, the output filter is not ideal, and the components of the filter may present non-linear properties depending on temperature, frequency, current etc. Therefore, the output filter causes often per se a distortion of the desired output signal.
It is possible to considerably reduce a number of the draw-backs associated with a conventional linear amplifier by using a class D amplifier, but such a class D amplifier does per se also involve problems raising the price of the power amplifier and causing undesirable characteristics, such as noise from the switch-frequency, distortions from the output filter, overvoltages etc.
DE-PS No. 198 38 765 discloses a power amplifier employing a hysteresis control for generating pulse width-demodulated voltages. The difference between the input voltage and the output voltage is integrated in this power amplifier, said difference being stepped down by a factor corresponding to the ratio of the maximum level of the input voltage to the maximum level of the output voltage. The difference between the scaled output signal and the input signal corresponds to the instantaneous amplitude error of the output signal with the result that the integration corresponds to the accumulated error on the output. The output signal of the integrator is triangular, and when the power amplifier is idle running, the slope of said triangle is of the same value for both the positive and the negative flanks. When the power amplifier is to be set, i.e. loaded, these flanks change in such a manner that the positive flank discloses a slope differing from the slope of the negative flank. However, the curve shape remains triangular with straight flanks. As the power amplifier is increasingly loaded, the switch-frequency decreases as well. As a result, for instance the input signal to the power amplifier is sinusoidal, and then the switch-frequency is at maximum at the zero-pass for the sine curve and significantly lower at the maximum and the minimum value, respectively, of said sine curve. When the power amplifier is loaded to its maximum, i.e. when the maximum value of the output voltage is almost identical with the internal DC-voltage of the power amplifier, then the switch-frequency becomes very low, almost zero. The triangular signal from the integrator is transferred to a comparator, typically a Schmitt-trigger, which converts the triangular signal into square pulses of a varying pulse width. These square pulses are the switching on signals and the switching off signals, respectively, for the transistors in the power amplifier. These switching on pulses are transferred to the output stage of the power amplifier, viz. to the transistors in the output, and therefore these pulses are upscaled by the relation between said pulse voltages and the internal DC-voltage of the power amplifier. The resulting voltage includes square pulses and is typically of a higher amplitude than the signal voltage. The square voltage is then transmitted to the output filter of the power amplifier, said output filter typically being a second order filter which is often referred to as a reconstruction filter. The voltage applying after the filter is the output voltage of the power amplifier. The voltage returned to the integrator is the voltage applying before the output filter. A modulator of this type is often referred to as an Astable Integrating Modulator or an AIM. Such a modulator is encumbered with the problem that the distortions of the output filter have not been taken into account. In addition, the operational amplifier used to construct the integrator has to be of high quality.
WO 02/25357 discloses a controlled oscillation modulator, also called a COM. The COM ensures that the open-loop-phase characteristics involve a phase shift of exactly 180° at the frequency where the open-loop-amplification is 0 dB. The latter is rendered possible by the feedback voltage from the output stage of the power amplifier being forwarded through function blocks causing a phase shift of 180° and/or through function blocks with time delays. The desired phase shift of 180° is obtained by including said phase shift in the function blocks, such as in form of a cascade coupling of poles, and/or by choosing a suitable time delay. When the feedback loop is subsequently closed, the modulator oscillates at the frequency where the amplification is 0 dB. When the input signal to the power amplifier is 0, the resulting signal is a substantially pure sine. When the input signal differs from 0, the oscillation is superimposed by the input signal. A comparator is subsequently used for generating the switching pulses of the output stage. An increasing loading of the amplifier has the effect that the pure sine resulting from the phase shift of 180° is altered into being something between the pure sine and the triangular voltage known from AIM. The linearity of a modulator depends on variations in the inclination of this signal. As this signal is not a pure triangular curve unlike AIM, but instead something between a sine curve and a triangular curve, the modulator according to the COM principle is nonlinear, and the modulation per se distorts the output signal.
WO 98/19391 describes a way of improving a power amplifier of the D class. The amplifier includes an internal modulator generating the well-known pulse-width-demodulated output signal. This signal is transmitted to an output filter, and the resulting filtered signal is the output voltage of the power amplifier. In order to compensate for the distortions of the filter, additional feedback loops have been included, and the characteristics of these feedback loops can compensate for the distortions of said output filter. The described system includes several cascade-coupled feedback loops for compensating the distortions. The system shows an improved procedure structure with respect to power amplifiers without such feedbacks, but the system is per se very complex and requires much design work in order to achieve the desired effect. A system of this type is often referred to as being Multivariable Enhanced Cascade Controlled or MECC.
U.S. Pat. No. 6,249,182 B1 discloses a modulator with an outer feedback loop after the output filter. The feedback has a lag-lead characteristic where the combination of the feedback, the output filter and the forward block creates a pole at zero, a double pole at the filter frequency and a zero followed by a pole in the feedback block.
U.S. Pat. No. 5,606,789 discloses a tracking converter comprising two Buck-converters. A discharge element ensures that the two converters are synchronized. The feedback is a current feedback, and the converter operates as a voltage controlled current generator.
U.S. Pat. No. 6,489,841 B2 discloses a switch made power amplifier, in which a resistor is placed in series with the capacitor of the output filter. This results in an output filter with two poles and a zero which reduces the suppression of noise. Furthermore, the power amplifier is AC-coupled and thus has poor amplification at low frequencies, Also, the poles f the output filter are far from the zero in the feedback and as a result, only one pole is achieved for the output filter.
U.S. Pat. No. 6,552,606 B1 discloses a modulator in which the feedback is the current measured through the capacitor of the output filter. The power amplifier is thus a voltage controlled current generator.
WO 03/090343 A2 discloses a power amplifier with a lead-lag feedback. The lead-lag in the feedback results in a second order response for the high frequencies.